The present invention relates to a process for producing a poly(arylene sulfide) (hereinafter referred to as PAS), and more in detail, the present invention relates to a novel process for producing a substantially linear PAS which contains an electrolytic alkali metal of not more than 50 ppm, shows a melt viscosity of not more than 300 poise (measured at 310.degree. C. and at a shear rate of 10,000 sec.sup.-1) and is suitable as a resin for packaging electronic parts.
Packaging of the electronic parts, for instance, IC, transistors, diodes, capacitors, etc. with a resin has been broadly carried out for the purpose of preserving the electric insulating property thereof, mechanically protecting the parts and preventing the change of the specific properties thereof due to the external atmosphere. Hitherto the packaging by the resins has been usually carried out by the transfermolding while using a thermosetting resin such as epoxy resin, silicone resin, etc.
As a packaging material, theremoplastic resins which show high productivity at the time of molding, heat-resistance, high fluidity and low content of electrolytes, are considered to be suitable.
Although the PASs represented by poly(phenylene sulfide) which have the properties of high fluidity and low melt viscosity, seem to be suitable for packaging material, PASs having a low content of the electrolytes have been very difficult to be obtained by the conventional process without subjecting to a certain special and complicated extraction. Accordingly, it is difficult to use the conventional PASs for the packaging materials which are vitally damaged by contamination of electrolytes.
In order to obtain a resin which is low in the content of the electrolytes, the extraction of a polymerized PAS with an organic polar solvent has been once tried. However, there is a problem that such an extraction is an economically significantly disadvantageous method and is not to be put to practical use.
For instance, a process for removing sodium chloride from a poly(phenylene sulfide) resin, comprising contacting the resin with a mixed solvent comprising a polar and aprotic solvent, and ethylene glycol [refer to Japanese Patent Application Laid-Open (KOKAI) No. 57-108135 (1982)], a process for purifying a poly(phenylene sulfide), which comprises heating the poly(phenylene sulfide) containing imprities comprising electrolytic components in an organic amide solvent in the presence of a polyoxyalkylene compound, thereby reducing the content of the electrolytic components [refer to Japanese Patent Application Laid-Open (KOKAI) No. 59-15430 (1984)] or so, have been proposed.
On the other hand, as for producing process of PAS, a process of reacting a dihaloaromatic compound with sodium sulfide in an organic amide solvent such as N-methyl-2-pyrrolidone or a process of adding a salt of a carboxylic acid to the said polymerization system has been conventional process [refer to Japanese Patent Publications No. 45-3368 (1970) and No. 52-12240 (1977)].
However, when PAS having a sufficiently low melt viscosity for exhibiting the high fluidity are tried to be obtained by these conventional producing process, a reaction mixture containing a large amount of the polymers of extremely low molecular weight and oligomers as well as a desired polymer having a properly low melt viscosity is obtained at the time of terminating the polymerization reaction. Accordingly, in PAS recovered from the reaction mixture, a large amount of the polymers of extremely low in molecular weight, oligomers and electrolytes, particularly alkali metals are mingled. Besides, the electrolytes are difficult to be removed from the PPS by the usual washing treatment and accordingly, it is difficult to obtain PAS of a low content of the electrolytes.
On the other hand, the present inventors have previously proposed, as a new process for producing a substantially linear PAS of a high melt viscosity by continuing the polymerization while adding water on the way of the polymerization and maintaining the polymerization temperature in high, that is, a new process for producing a high molecular weight PAS of a melt viscosity of more than 1,000 poise (measured at 310.degree. C. and at a shear rate of 200 sec.sup.-1), wherein PAS is obtained by reacting an alkali metal sulfide with a dihaloaromatic compound in an organic amide solvent.
More in detail, the proposed process comprises the steps of
(1) carrying out the reaction at a temperature of from 180.degree. to 235.degree. C. in the presence of from 0.5 to 2.4 mol of water per mol of the alkali metal sulfide, thereby forming a PAS of a melt viscosity of from 5 to 300 poise at a conversion rate of the dihaloaromatic compound of from 50 to 98 mol % and PA0 (2) continuing the reaction by adding water so that from 2.5 to 7.0 mol of water per mol of the alkali metal sulfide is present and by raising the temperature of the reaction system to from 245.degree. to 290.degree. C. [refer to Japanese Patent Application Laid-Open(KOKAI) No. 61-7332(1986)]. PA0 [A] reacting an alkali metal sulfide with from 1.0 to 2.0 mol of a dihaloaromatic compound per mol of the alkali metal sulfide at a temperature of from 180.degree. to 235.degree. C. in a state wherein from 0.5 to 2.4 mol of water per mol of the alkali metal sulfide is present so that the conversion rate of the dihaloaromatic compound is not less than 50% of the theoretically necessary amount and the melt viscosity (measured at 310.degree. C. and at a shear rate of 10,000 sec.sup.-1) of the thus formed PAS is less than 5 poise, PA0 [B] continuing the reaction while adding water to the reaction system so that from 2.5 to 7.0 mol of water per mol of the alkali metal sulfide is present in the reaction system and raising the temperature of the reaction system to from 250.degree. to 290.degree. C. so that the melt viscosity (measured at 310.degree. C. and at a shear rate of 10,000 sec.sup.-1) of the thus formed PAS is not less than 5 times of that of the PAS formed in the step[A] but not more than 300 poise, and PA0 [C] after separating the PAS formed in the step[B] from the polymerization reaction mixture, treating the thus separated PAS in an aqueous solution (pH: less than 2) of a strong acid or an aqueous 0.1 to 30% by weight solution of a salt of a weak base and the strong acid for from 1 to 500 minutes at a temperature of from 0.degree. to 200.degree. C., thereby obtaining a PAS of an extremely low content of the alkali metal and of a low melt viscosity without subjecting the thus formed PAS to a special extraction treatment or washing treatment, because the specific surface area of the particles of the thus formed PAS is large enough to be easily washed.
However, although the thus proposed process is a suitable process for producing a substantially linear PAS of a high melt viscosity, it is not necessarily a suitable process for producing a PAS of a sufficiently low melt viscosity for exhibiting a high fluidity.
As a result of the present inventors' extentive studies for producing a PAS of a low content of the electrolytes and of a high fluidity, a process has been found by the present inventors whereby a PAS of a low content of the electrolytes and of a low melt viscosity can be produced economically, and on the basis of the finding, the present invention have been attained.